1. Field of the Invention
The present invention relates to a process for the separation of sulfur compounds from hydrocarbon mixtures using a membrane.
2. Background of the Invention
Sulfur compounds are impurities in gasoline that compromise vehicle emission controls by poisoning the catalytic converter. In an effort to further decrease emissions, the U.S. government has recently proposed a nationwide reduction of sulfur in gasoline from current levels at 300-1000 ppm to an average of 30 ppm (Federal Register, 64(92), May 13, 1999). Gasoline producers, both domestic and foreign, selling fuel in the U.S. would be expected to comply by the year 2004.
Presently, the conventional process for reducing sulfur content in gasoline involves hydrotreating in which sulfur compounds are converted to volatile hydrogen sulfide and other organics. This energy intensive process, requiring elevated temperature and pressure, is expensive for obtaining the proposed lowered sulfur levels. Alternative processes with more efficient sulfur-reducing technology are needed to maintain progress toward cleaner burning fuels.
The use of membrane separation technology, in which select compounds or types or compounds can be separated from an organic mixture by permeation through a membrane, has been reasonably well developed. Separation processes that incorporate membranes present an attractive option for large-scale purification of petroleum fractions because of their inherent simplicity, versatility, and low energy consumption.
Typically, membrane separation processes rely on the affinity of a specific compound or class of compounds for the membrane. In this way, the components of a mixture with specific affinity for the membrane will selectively sorb onto the membrane. The sorbed compounds diffuse, or permeate, through the membrane and are removed on the opposite side. Continual withdrawal of permeated compounds from the membrane maintains the driving force for the separation process. Removal of permeated compounds is usually achieved by pervaporation or perstraction methods. Pervaporation employs a vacuum on the permeate side of the membrane, removing the permeated compounds in gaseous form, while perstraction employs a liquid sweep stream, continually washing away permeate.
The chemical properties of the membrane dictate the type of compound that has affinity for it. Some types of membranes are composed of charged chemical groups and are, therefore, considered ionic in character. An example of an ionic membrane is Nafion(copyright) (available from DuPont, of Wilmington, Del.), which is a polymer of perfluorosulfonic acid that has been used principally in the dehydration of liquid organic mixtures as described in U.S. Pat. No. 4,846,977. Only few examples exist for the use of Nafion(copyright) in separating organic compounds. U.S. Pat. No. 4,798,764 describes the separation of methanol from dimethyl carbonate or methyl t-butyl ether. The use of Nafion(copyright) membranes for the separation of mixtures of styrene and ethylbenzene has also been reported (Cabasso, Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 313). U.S. Pat. No. 5,498,823 reports the enhanced separation of unsaturated organic compounds using silver ion-exchanged Nafion(copyright) membranes. A related ionic membrane composed of sulfonated polysulfone has been also used for the separation of aromatics and non-aromatics as disclosed in U.S. Pat. No. 5,055,631. To date, the use of ionic membranes, such as Nafion(copyright), for the separation of sulfur compounds from liquid organic mixtures has not been reported.
In contrast to ionic membranes, non-ionic membranes are made from those materials lacking charged chemical groups. Chemical affinity for these membranes is usually governed by the hydrophilic or hydrophobic nature of the membrane material. Hydrophilic membranes have affinity for water or other polar compounds, and those membranes with affinity for water are often water-soluble. Hydrophilic membranes include both ionic and non-ionic membranes. However, the non-ionic membranes generally contain polar chemical groups such as hydroxyl, carboxyl, sulfonyl, carbonyl, or amine groups. Examples of hydrophilic non-ionic membranes include polyvinylalcohol (PVA), cellulose acetates, and polyvinylamine. Hydrophobic membranes, on the other hand, have little affinity for water or polar compounds and generally lack or contain a small proportion of charged or polar chemical groups. Examples of hydrophobic membranes include polyethylene and polystyrene.
A wide variety of non-ionic membranes have been used in separation processes. U.S. Pat. Nos. 5,905,182, 5,019,666, 4,997,906, 4,944,880, 4,532,029, 4,802,987, 4,962,271, 5,288,712, 5,635,055, 3,556,991, 3,043,891, and 2,947,687 describe the separation of aromatics from hydrocarbon mixtures using a wide variety of non-ionic membrane materials. Non-ionic membranes have also been used in the separation of aromatics containing heteroatoms from hydrocarbon mixtures as disclosed in U.S. Pat. Nos. 5,643,442 and 5,396,019. The aforementioned patents, which are incorporated herein by reference, disclose membrane separation processes directed to the separation of aromatics and non-aromatics using hydrophobic membranes.
The proposed mandate for lowered sulfur levels in gasoline has made it imperative to improve or replace existing methods for desulfurization of petroleum fractions. A more cost-effective method for reducing sulfur content in petroleum fractions is a primary goal of the oil refining industry. Current membrane separation technology shows potential for meeting future standards, but has not yet been used specifically for this purpose.
This invention relates to a process for the separation of sulfur compounds from hydrocarbon mixtures, preferably oil refining fractions, using a membrane. The membrane may be composed of any material, ionic or non-ionic, that preferentially permeates sulfur compounds over hydrocarbons. The hydrocarbon mixture is split by one or more membranes forming sulfur-rich and sulfur-lean fractions. The sulfur-lean fraction may be incorporated into fuel mixtures and the sulfur-rich fraction may undergo further treatment for reduction of sulfur levels.
The present invention provides a process for separating sulfur compounds from a hydrocarbon mixture containing at least one sulfur compound and hydrocarbons comprising the steps of:
(a) contacting said hydrocarbon mixture with a first compartment of a membrane module, said membrane module further comprising a second compartment and a hydrophilic membrane separating said first compartment and said second compartment;
(b) selectively permeating said sulfur compounds of said hydrocarbon mixture through said membrane such that a sulfur-rich fraction accumulates in said second compartment and a sulfur-lean fraction is retained in said first compartment; and
(c) retrieving said sulfur-rich fraction from said second compartment and said sulfur-lean fraction from said first compartment.
The present invention also provides a process which further comprises the steps of:
(d) contacting said sulfu-rich fraction of step (c) with a first compartment of a further membrane module, said further membrane module comprising a second compartment and a hydrophilic membrane separating said first compartment and said second compartment;
(e) selectively permeating sulfur compounds of said sulfur-rich fraction of step (d) through said membrane such that a further sulfur-rich fraction accumulates in said second compartment and a further sulfur-lean fraction is retained in said first compartment; and
(f) retrieving said further sulfur-rich fraction and said further sulfur-lean fraction;
(g) repeating steps (d), (e) and (f) using said sulfur-rich fraction until a final sulfur-rich fraction of desired sulfur content is obtained; and
(h) retrieving said final sulfur-lean fraction.